I’m not sure they’re actually separate species – Wikipedia tells me those scientific names are synonymous. Offhand, I know the American goldfinch was recently reclassified into the genus Spinus from Carduelis, so the same thing might have happened to the pine siskin.

I suspect a combination of camera angle/lighting and individual plumage variation accounts for the difference you’re seeing, because I can’t find any source that says there are two separate eastern and western pine siskin species.

No, there’s only one species, Linda. The different generic names represent different taxonomic assignments at different times. When I started birding in the 70s, siskins were considered to constitute a distinct genus, Spinus, and the Pine Siskin was Spinus pinus.

Then, I think back in the 90s, Spinus was merged into the genus Carduelis, and the Pine Siskin became Carduelis pinus, but placed in the subgenus Spinus.

Still later on (in 2009), based largely on mitochondrial DNA evidence, the siskins were once again recognized as generically distinct, and the subgenus Spinus was re-elevated to generic rank.

This kind of thing happens all the time. Taxonomy isn’t static, and taxonomists–creative bunch that they are–are constantly developing new techniques to bring to bear on the thornier questions.